In impact printing, a hammer like device is used to impart a slug carrying a print font against an ink carrier and a record medium, such as ordinary paper, which is backed by a platen. The platen is an anvil for the hammers blow. The ink carrier is conventionally a ribbon, i.e. an elongated web impregnated with ink. The ink is transferred to the paper record medium when the two are brought into intimate contact and by the blow of the hammer. Ink is released from the ribbon in raised areas in the slug corresponding to the shape of the character font. Broadly, a serially impact printer is one in which a line of print is inscribed one character at a time. Classic examples of serial printers are the familiar office typewriter, teletypewriter printers, and low speed computer output printers. Other classic impact imprint printers include calculating machines, such as adding machines, and business accounting machines which use mostly numerical characters.
Printing elements for serial printers shaped generally in the form of a wheel or the like have been known for some time. U.S. Pat. No. 3,954,163 describes a serial printer which has a print wheel having a plurality of slugs located at the ends of spokes or beams extending radially outward from the hub. The print wheel is rotated by a servo mechanism to position selected characters opposite a hammer and ribbon at the printing station. The print wheel employed is basically a single element structure in which the beams and slugs are integrally molded thermoplastic structure. This print wheel delivers superior performance with very favorable economics i.e. the integral wheel is relatively inexpensive to manufacture. Nevertheless, once subjected to long time useage or useage where high print qualities are required, the integral structure print wheel does not always give the desired print quality.
Specifically the molded single piece plastic print wheel has elements in its structure which require different mechanical properties. The spokes must be flexible so that when they are struck, they readily bend forward to strike the ribbon and paper and immediately return to the home position. The character font on the other hand must be very hard and not deform under impact. As a result of these, somewhat opposite requirements the print wheels are essentially a compromise of properties required for the spokes and the character fonts. Over continued use the character fonts tend to either wear away or have portions break off both of which lead to unacceptable print quality after only a relatively short servicable life. It is not uncommon, for example, for a print wheel to have a servicable life of less than one week in a high use environment. In addition, for those applications requiring very high print quality higher energy impaction of the print slug is required. This tends to make the font spread out and disfigure.
One approach in the design of print wheels to provide wheels which withstand greatly increased impact force from the hammer is the composite print wheel described in U.S. Pat. No. 4,037,706. In this composite print wheel the spokes or beams are fabricated from a thin sheet of metal to give them flexibility and strength, and the character slugs or fonts are molded onto the ends of the spokes or beams. The body portion of the slugs in that composite print wheel may be made of such polymeric material as fiberglass reinforced phenolic resin, which is thereafter plated with a metallic wear resistant coating such as a nickel alloy. In this fashion, the beams are like leaf springs which vibrate or oscillate when deflected toward a print station and which quickly retreat back toward their initial position. On the other hand the slugs are made of a hard, wear resistant high modulus material to withstand the impact of the hammer to give high print quality over long periods of time.
Although the composite print wheel of said U.S. Pat. No. 4,037,706 gives high print quality over long periods of time, it is relatively expensive and time consuming to make. Thus there is a need for a print wheel which is economical to make and yet is capable of giving high print qualities over long periods of time while being subjected to the high impact force of modern serial printers.
Accordingly, it is an object of the present invention to provide a print wheel having a long useful life.
It is another object of the present invention to provide a relatively inexpensive integral print wheel.
It is a further object of the present invention to provide an integral print wheel where the character fonts and the spokes have different mechanical properties.
It is a further object of the present invention to provide a method for making polymeric print wheels having hardened and improved character fonts without adversely affecting the flexibility and strength of the spokes or beams.
It is a further object of the present invention to provide a method of modifying the mechanical properties of selected portions of a polymeric structure.
Other objects of the invention will be appreciated from the following description of the invention.